The present invention relates to a friction stir welding method that is particularly preferable for joining aluminum alloy members to form transportation devices such as railway cars, industrial equipment, science-related equipment, electric appliances, and so on.
Friction stir welding is a method performed by inserting a rotating shaft (called a rotary tool) to the joint portion between members to be welded and moving the rotary tool along the joint line, thereby heating, softening, plasticizing and solid-phase welding the joint portion. The rotary tool comprises a large-diameter portion and a small-diameter portion. During welding, the small-diameter portion is inserted to the member(s) to be welded, and the end surface of the large-diameter portion comes into contact with the member(s). A screw thread is formed to the small-diameter portion. The end surface of the large-diameter portion facing the small-diameter portion is sloped and concaved. The central axis of the rotary tool is tilted in the direction of movement of the rotary tool. That is, the axis of the tool is tilted rearward to the direction of movement thereof. In another example, a bobbin-type rotary tool is used to friction stir weld a member placed between the two large-diameter portions of the bobbin-type tool.
According to another example, projections protruding to the side from which the rotary tool is inserted are formed to the two members to be welded, and friction stir welding is performed by inserting the small-diameter portion of the rotary tool to the abutted portion and inserting the large-diameter portion of the rotary tool to the projections. The metal material constituting the projections is used to fill the gap formed between the two abutted members. The projection can be formed only to one of the two members to be welded. Such method is used to weld extruded members, the friction stir welding being performed to the portion where the extruded direction of one member is orthogonal to that of the other member.
The above mentioned prior art methods are disclosed in the publications of Japanese Patent No. 3070735 (U.S. Pat. No. 6,237,829), Japanese Patent No. 2712838 (U.S. Pat. No. 5,460,317), Published Japanese Translation of PCT Patent Application No. 508073/97 (EP 0752926B1), and Japanese Patent No. 3070735 (U.S. Pat. No. 6,237,829).
When performing the friction stir welding method, if only one of the two members being abutted for welding has the projection as mentioned above, it is difficult to obtain a good weld as compared to the case where both members are provided with the projection.
If a gap exists at the abutted portion, the metal constituting the projection(s) is used as the material to fill the gap formed thereto. However, if only one of the two members being abutted has the projection, it is difficult to obtain a good weld because of the gap existing at the abutted portion. Even if both abutted members are provided with the projection, it becomes difficult to obtain a satisfactory weld as the gap increases.
Moreover, since the central axis of the rotary tool must be tilted in the direction of movement of the rotary tool, if the joint line is curved, it is necessary to control the tilt of the axis to a predetermined angle in the direction of movement of the tool, which involves complicated operation. Especially when the radius of curvature of the joint is small, the tilt control becomes even more difficult.
The object of the present invention is to provide a good friction stir weld in cases where only one of the members being abutted for welding has a projection, where a large gap exists at the abutted portion, or where the joint line is curved.
The object of the present invention is achieved by a friction stir welding method comprising abutting a pair of members, wherein only one of the members has a projection formed at the abutted portion thereof, the other member being abutted to the portion excluding the projection of the first member, using a rotary tool having protruding and/or recessed portions formed to the end surface of a large-diameter portion that faces a small-diameter portion, the small-diameter portion being inserted in the abutted portion and the end surface of the large-diameter portion being inserted only in the projection during the friction stir welding.
The object of the present invention is achieved by utilizing a rotary tool having on the end surface of the large-diameter portion facing the small-diameter portion a groove that extends from the outer periphery of the large-diameter portion and reaches the small-diameter portion side, and while inserting the end surface of the large-diameter portion and the small-diameter portion to the members, rotating the rotary tool so that the metal material of the member(s) move along said groove toward the axial center of the rotary tool.
The object of the present invention is achieved by utilizing a rotary tool comprising a large-diameter portion and a small-diameter portion, and performing the friction stir welding method while the small-diameter portion and the end surface of the large-diameter portion are inserted to the members. The large-diameter portion comprises a first large-diameter portion formed as a separate member from the small-diameter portion, and a second large-diameter portion that is formed as the same member as the small-diameter portion, wherein the small-diameter portion, the first large-diameter portion and the second large-diameter portion are arranged in this order along the central axis of the rotary tool, the first large-diameter portion being screwed onto the axial portion between the second large-diameter portion and the small-diameter portion, the end surface of the first large-diameter portion being in contact with the end surface of the second large-diameter portion, the rotary tool being rotated in the same direction as the screw thread used to screw the first large-diameter portion to the axial portion.